As the integration density of integrated circuit memory devices (i.e., semiconductors) increases, the capacitance of associated capacitor therein can be increased by reducing the thickness of an equivalent oxide (Tox) of a dielectric film while maintaining a relatively low leakage current. However, in the case of an Metal-insulator-Semiconductor (MIS) structure, a surface of a polysilicon lower electrode may be oxidized by air to form an oxide film thereon. Accordingly, the thickness of the equivalent oxide Tox of the dielectric film may increase, thereby causing a leakage current at the interface between the lower electrode and the dielectric film.
The above problem has been addressed by using a high dielectric material, decreasing the capacitance constant and using a relatively high work function metal film (such as TiN, TaN, or W) rather than polysilicon for the lower electrode. When the lower electrode is formed of such a metal, the thickness of the equivalent oxide Tox of the dielectric film may be reduced, and the leakage current may also be decreased.
Typically, the lower electrode of a capacitor in a memory device is electrically coupled to a source (or contact pad) of a MOS transistor via a contact plug, which generally is a doped polysilicon film. When the lower electrode (made of one of the above metals) is formed on the polysilicon contact plug, the contact resistance may be increased due to the lack of an ohmic contact between the lower electrode and the contact plug. In this case, the surface of the polysilicon contact plug may be oxidized, thereby increasing thickness of the equivalent oxide Tox.
The above problem associated with oxidation has been addressed by replacing the polysilicon contact plug with a metal contact plug. However, a metal contact plug and the polysilicon plug are typically formed using different steps. Accordingly, forming the contact plug as a metal contact plug may require additional manufacturing equipment. For example, in some situations polysilicon contact plugs and resistor (that can act signal delays in a semiconductor memory device) can be formed during the same step in manufacturing process. However, if the contact plug is formed of metal, the process for forming the metal contact plug and the process for forming the resistor may need to be performed separately, thereby, possibility complicating the process by, for example, adding photolithography steps to the process.